Hormonal Control of the Menstrual Cycle

Coordinating the ovaries and uterus

Successful reproduction requires more than releasing an egg: the uterus must also be ready for a developing embryo to implant in its lining. The menstrual cycle coordinates these changes in the ovaries and uterus.

Hormones are chemical messengers produced by glands and carried in the blood. During the menstrual cycle, hormones from the pituitary gland, at the base of the brain, act on the ovaries. Hormones from the ovaries act on the uterus and also influence further hormone release from the pituitary gland. This allows different parts of the body to work together.

The sequence of changes

A typical menstrual cycle lasts about 28 days, although its length can vary. Day 1 is the first day of menstruation, when the uterus lining breaks down and is shed through the vagina. Menstruation commonly lasts around five to seven days.

After menstruation, the uterus lining rebuilds and thickens. Meanwhile, an egg matures inside a structure called a follicle in an ovary. Around day 14 of a typical 28-day cycle, ovulation releases the egg from the ovary. The egg then travels along the oviduct towards the uterus.

After ovulation, the uterus lining is maintained, ready for implantation if fertilisation occurs and an embryo develops. If pregnancy does not occur, the lining is shed and another cycle begins.

FSH starts follicle development

The pituitary gland releases follicle-stimulating hormone (FSH). FSH stimulates a follicle and the egg inside it to mature in an ovary. It also stimulates the developing follicle to produce oestrogen.

This is the first important hormone interaction: FSH acts on the ovary, causing it to produce another hormone. The effects of FSH therefore help coordinate egg development with changes in the uterus.

Oestrogen rebuilds the lining and helps trigger ovulation

Oestrogen causes the uterus lining to repair and thicken after menstruation. It also inhibits FSH release from the pituitary gland. Inhibiting means reducing or preventing release; this limits further stimulation of follicles as the cycle progresses.

When oestrogen reaches a high level before ovulation, it stimulates the release of luteinising hormone (LH) from the pituitary gland. The rise in LH triggers ovulation.

Oestrogen therefore has different effects on the two pituitary hormones: it inhibits FSH, but a high level stimulates LH. The sequence is follicle development, rising oestrogen, increased LH and then egg release.

Progesterone maintains the lining after ovulation

After the egg has been released, the follicle left behind becomes the corpus luteum. This structure produces progesterone.

Progesterone maintains the thickened uterus lining, keeping it suitable for implantation. It also inhibits the release of both FSH and LH from the pituitary gland. This reduces further follicle development and prevents another ovulation while the uterus is prepared for a possible pregnancy.

The hormone interactions form a coordinated control system. Some signals stimulate the next event, while others inhibit hormone release. These interactions keep egg development, ovulation and preparation of the uterus in the correct sequence.

Falling progesterone starts another cycle

If the egg is not fertilised, the corpus luteum breaks down and progesterone levels fall. Without progesterone maintaining it, the uterus lining breaks down: menstruation begins again.

The fall in progesterone also removes its inhibition of FSH release, allowing another follicle to develop. The cycle can therefore repeat.

If pregnancy occurs, progesterone production continues and the uterus lining is maintained rather than shed. Later, the placenta produces progesterone to help maintain the pregnancy.

Hormone sources and effects

Recall the sequence

FSH → follicle matures → oestrogen rises → high oestrogen stimulates LH → ovulation → corpus luteum produces progesterone → lining maintained.

If no fertilisation occurs: corpus luteum breaks down → progesterone falls → menstruation; FSH inhibition is removed and another cycle begins.

Timing and distinctions

• Day 1: menstruation begins.
• Typical cycle: about 28 days; ovulation around day 14.
• Ovulation: egg released from an ovary.
• Menstruation: uterus lining shed.
• Oestrogen rebuilds the lining; progesterone maintains it.